Our Iongterm objective is to understand the interactions of the innate and adaptive immune response in the pathogenesis of graft rejection. Recent work from our laboratory investigating cardiac transplants in lymphocyte deficient recipients has characterized a robust innate (antigen independent) immune response following transplantation including the identification of a candidate gene, interleukin 6 (IL-6) that is produced by graft tissue, is markedly upregulated by ischemia, and is necessary for robust graft rejection. Interestingly, data from our murine model shows that IL-6 deficiency in the transplant recipient does NOT prolong graft survival, whereas IL-6 deficiency in the donor graft tissue significantly prolongs graft survival more than 3 fold. Although IL-6 is produced within the graft, major increases in IL-6 receptor (both IL-6Ralpha and gp130) occur in the lymph node, but not within the graft. Based on these observations, we postulate that the targets for IL-6 dependent effects could be lymph node cells, suggesting that IL-6 functions at a distance by endocrine like mechanisms. Consistent with this possibility, we also detect high levels of IL-6 in the serum in the IL-6 deficient recipient group, but not in the IL-6 deficient graft group. Relevant to this paradigm, recent studies have established that IL-6 can block immune suppression by regulatory T cells (Treg). Thus, we will investigate links between graft produced IL-6 and functions of Treg cells following transplantation. Most previous studies investigating rejection have focused on the immune response versus the graft. In contrast, our proposal will focus on the proinflammatory response induced by the graft tissue that we postulate is a crucial component in promoting the adaptive rejection response. Thus, an innate response generated by the graft tri.q.qers an adaptive alloimmune response. Our hypothesis is that IL-6 is produced by the graft by antigen-independent innate immune mechanisms and enhances the adaptive alloimmune response that mediates rejection by inhibiting regulatory T cell function. Aim 1 will investigate the production and regulation of IL-6 by graft tissue following transplantation. Aim 2 will identify cells in the peripheral lymphoid tissue with IL-6 dependent activation. And, Aim 3 will determine the mechanisms of IL-6 dependent activation of alloimmune responses by modulation of regulatory T cells.